5.4 Cladistics

A. Value of Classifying Organisms 1. Organization of data assists in identification 2. Suggests evolutionary links 3. Allows prediction of characteristics shared by members of a group

B. Biochemical evidence of evolution 1. Universality of the genetic code a. All amino acids are coded for by mRNA codon sequences, that are transcribed from DNA codons b. codons are derived from the same 4 bases regardless of species, A, U, C, G

c. The universality of the code points to a common evolutionary ancestry

2. Hemoglobin a. Found in most animals, but the nucleotide sequence can vary by species B. Tracking and comparing these variations can help species relative to each other on the phylogenetic tree

Similarities in Amino Acid Sequence of Cytochrome C

Phylogeny The system of classifying organisms based on evolutionary origins and changes over time Changes in DNA can help construct a phylogenic tree Show stems and branches that indicate evolutionary relationships among organisms

3. Mutations can be used as an “Molecular Clock” a. Mutations in a genome (due to mistake in replication) occur at a predictable rate. b. Therefore, base pair sequences in two populations can be compared by looking at the number of differences. You can make an inference as to how long the 2 populations diverged reproductively

c. the greater the number of differences, the farther apart the two groups are on the phylogenetic tree.

Another example

C. Cladistics 1. Clade =a group of organisms who share common ancestry 2. Cladistics = A taxonomic system of separating clades based on sharing derived characteristics from common ancestry

Sample Cladogram

3. Analogous Vs. Homologous characteristics a. Analogous characteristics show similarity without having a common ancestor Develop due to evolving in similar habitats or facing similar environmental challenges E.g. the wings of bats, birds and insects

Analogous Characteristics

b. Homologous characteristics show similarity due to having a common ancestor E.g bone structure of a whale flipper and a human hand. Used for developing cladograms

4. How to Create a Cladogram a. Start with an “in group”, which contain certain characteristics b. Another groups is then compared to the “in group” c. If it differs in any way it is placed in its own clade d. Clades are separated from each other based on single differences, and then placed in sequence

e. Note cladograms do not make any assumptions about the time period involved in an evolutionary change, rather they indicate one has occurred

Sample Organisms & Characteristics B C D Multicellular X Backbone 4-chamber heart Wings

5. Relationship between cladograms & the classification of living organisms a. Monophyletic – group that shares a common ancestor

b. Paraphyletic –a group which contains some, but not all members associated with a common ancestor.

c. Polyphyletic – a group which does not share a common ancestor

Reclassification of Cladograms Traditional classification based on morphology doesn’t always match evolutionary origins (DNA evidence) Some groups have been reclassified Some groups have merged Other groups have been divided Some species have been transferred from one group to another